It's also one of those things that actually is simple in restricted cases, then when you try to generalise it blows up badly, and it blows up like a time bomb rather than a landmine. When a time bomb explodes you don't quite know when it was triggered, but you have to deal with it then and there, learning a heck of a lot about time libs in general before you can proceed. What's a leap second, which years have a leap day, are time zones connected to places somehow, and so on. Just so you can fix your calendar that's part of a larger app.